Instruments for providing a representation dependent upon the value of a predetermined variable



June 25, 1968 J. R. HOLTAM 3,389,605

INSTRUMENTS FOR PROVIDING A REPRESENTATION DEPENDENT UPON THE VALUE OF A PREDETERMINED VARIABLE Filed Oct. 19, 1966 4 Sheets-Sheet l JOHN Ric M p Hmwr J. R. HOLTAM 3,389,605 INSTRUMENTS FOR PROVIDING A REPRESENTATION DEPENDENT June 25, 1968 UPON THE VALUE OF A PREDETERMINED VARIABLE Filed Oct. 19, 1966 4 Sheets-Sheet 2 JOHN RICK/4RD HOLT/W/ June 25, 1968 J. R. HOLTAM 3,389,605

INSTRUMENTS FOR PROVIDING A REPRESENTATION DEPENDENT UPON THE VALUE OF A PREDETERMINED VARIABLE Filed Oct. 19, 1966 4 Sheets-Sheet 3 JOHN R/ckARp Hm/w/ June 25, 1968 J. R. HOLTAM 3,339,605

INSTRUMENTS FOR PROVIDING A REPRESENTATION DEPENDENT v UPON THE VALUE OF A PREDETERMINED VARIABLE Filed Oct. 19, 1966 4 Sheets-Sheet 4 United States Patent "ice 3,389,605 INSTRUMENTS FOR PROVIDING A REPRESENTA- TION DEPENDENT UPON THE VALUE OF A PREDETERMINED VARIABLE John Rickard Holtam, Stroud, England, assignor to Smiths Industries Limited, London, England, a British company Filed Oct. 19, 1966, Ser. No. 587,888 Claims priority, application Great Britain, Oct. 19, 1965, 44,263/65 16 Claims. (Cl. 73-486) This invention relates to instruments for providing a representation dependent upon the value of a predetermined variable.

According to the present invention an instrument for providing a representation dependent upon the value of a predetermined variable, comprises a member that is arranged for angular displacement about each of two mutually perpendicular axes, a sensor that is responsive to the value of said variable to apply to said member drive having moment about each of the two axes, means for deriving said representation from angular displacement of said member about a first of the two axes, and servo means arranged to drive said member about said first axis in response to angular displacement of said member about the second axis such as to reduce this latter displacement to zero and assist the sensor in driving said member to an angular displacement about the first axis in accordance with the sensed value of said variable.

The said member may be a rotatably-mounted shaft, and in this case said first axis is preferably the longitudinal axis of the shaft, means for providing said representation (for example, a pointer and, or alternatively, a digital counter) being couple-d to the shaft through gearing, so as to be driven in accordance with the angular displacement of the shaft about its longitudinal axis. The gearing in this context may be provided in part at least by mechanical links as well as by gears, and drive from the servo means may be applied to the shaft back through at least part of this gearing.

It may be arranged that in the event of the servo means being inoperative to apply drive in response to angular displacement about the second axis, said member is restrained from angular displacement about the second axis, the instrument functioning in these circumstances to provide said representation as before but without the assistance of the servo means in achieving the appropriate angular displacement of said member about the first axis. In this connection, where the servo means is electrically powered, it may be arranged that the said member is restrained from angular displacement about said second axis as aforesaid, in response to failure of the electrical power supply to the servo means; for example, a solenoid may be arranged to hold a locking device, against the action of a spring, out of engagement with said member only while the electrical power is supplied, the locking device being released to engage said member positively under the action of the spring when power-supply is broken. Furthermore, it may be arranged that, in addition to said member being restrained from angular displacement about said second axis in the event of the servo means becoming inoperative as aforesaid, such servo means is in this event decoupled from said member so as thereby to reduce opposition to angular displacement of said member about said first axis.

The instrument may be an altimeter, the sensor in this case being a manometric capsule arranged to be deflected in accordance with barometric air-pressure. In this respect, the invention is especially applicable to altimeters for use in aircraft, and according to a feature of the present invention, an altimeter comprises a manometric 3,389,605 Patented June 25, 1968 capsule for providing deflection in accordance with barometric air-pressure, a shaft that is mounted for angular displacement about its longitudinal axis and also about a second axis normal to the longitudinal axis, indicator means coupled to the shaft for providing a visual representation of altitude in accordance with the angular displacement of the shaft about its longitudinal axis, means coupling the capsule to the shaft such as to tend to displace the shaft angularly about its longitudinal axis in accordance with the deflection of the capsule, the thrust applied to the shaft by the latter means having moment about said second axis as well as about the longitudinal axis, and an electrical servo arrangement that is arranged to be responsive to angular displacement of the shaft about said second axis to drive the shaft about its longitudinal axis in a sense to reduce to zero angular displacement of the shaft about said second axis and thereby to tend to maintain the angular displacement of the shaft about its longitudinal axis in accord with the barometric pressure sensed by the capsule.

The altimeter preferably includes locking means that is arranged to be released to urge the shaft to, and then lock it in, a datum position of zero angular displacement about the second axis in the event of break of supply of electrical power to the servo arrangement. The altimeter in this case is adapted to continue to provide visual representation of altitude in spite of break in the supply of electrical power; such a feature is clearly of considerable merit from the point of view of safety.

According to another feature of the present invention an altimeter comprises two rocker shafts each of which is mounted for angular displacement about its longitudinal axis, two manometric capsules that are responsive to barometric air-pressure and are mounted backto-back between the two rocker shafts, two mechanical linkages coupling the two capsules to the two shafts respectively to tend to displace each shaft angularly about its longitudinal axis in accordance with the barometric air-pressure, the thrusts applied to the shafts via the link ages having moments about the transverse axes as well as about the longitudinal axes of the two shafts, indicator means coupled through gearing to both rocker shafts to provide representation of altitude in accordance with the angular displacements of the shafts about their longitudinal axes, and an electrical servo arrangement for driving the two rocker shafts about their longitudinal axes in response to angular displacements of the shafts about their transverse axes and such as to reduce these latter displacements to zero.

Two altimeters for use in aircraft, and both in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 is a schematic representation in perspective of a first of the altimeters;

FIGURE 2 illustrates in end elevation, and partly in section, the manner in which two manometric capsules of the second altimeter are each coupled to an individual rocker shaft;

FIGURE 3 is a plan view of part of the second altimeter, the end elevation of FIGURE 2 being taken on the line IIII of FIGURE 3;

FIGURE 4 illustrates in end elevation taken on the line IVIV of FIGURE 3, the manner in which the rocker shafts are both coupled to a common output shaft of the second altimeter;

FIGURE 5 is an end elevation taken on the line VV of FIGURE 3; and

FIGURE 6 shows schematically and in perspective the form of the second altimeter.

Referring to FIGURE 1, an evacuated manometric capsule 1 that is responsive to barometric air-pressure is coupled through a link 2 to a crank arm 3 of a rocker shaft 4 so as to produce displacement of the shaft 4 in accordance with deflection of the capsule 1. The rocker shaft 4 has a second crank arm 5 that is coupled via a link 6 to drive a sector gear 7 in accordance with displacement of the shaft 4. The gear 7, in its turn, drives a pinion 8 that is coupled via a differential gear 9 to both a pointer 10 and a digital counter 11 in the altimeterdisplay, so that the angular deflection of the pointer 10 in the display and the digital representation provided by the counter 11 are both dependent upon the deflection of the capsule 1 and, accordingly, upon altitude. The representations provided by the pointer 10 and counter 11 are both adjustable manually by means of a knob 12 that is coupled to the pointer 10 and counter 11 through the differential gear 9, this enabling the two representations of altitude to be corrected in accordance with variation in datum pressure, that is to say, in the barometric pressure obtaining at ground-level.

The deflection of the capsule 1 resulting from change in altitude acts upon the arm 3 via the link 2 so as to tend to produce angular displacement of the shaft 4 about its longitudinal axis. The shaft 4 is mounted in two jewel bearings 13 and 14 at either end, to facilitate such displacement. The bearing 13, which is the nearer to the crank arm 5, is secured to the support framework of the altimeter in a conventional manner, whereas the bearing 14 is mounted on the framework by means of a pair of parallel leaf-springs 15. The leaf-springs 15, which extend laterally at right-angles to the shaft 4, allow for a small degree of movement of the bearing 14 in either sense normal to these springs 15. The shaft 4 in these circumstances has limited freedom for angular displacement with the bearing 14 (in either sense, as indicated .to an exaggerated extent by arrows 16) about an axis 17 which is parallel to the leaf-springs at the fixed hearing 13, and which, accordingly, is normal to the longitudinal axis of the shaft 4.

A pick-off 18 is responsive to any angular displacement of the shaft 4 about the axis 17 to supply an electric signal that is dependent upon the displacement to a servo amplifier 19. This signal provides a measure, in magnitude and sense, of the angular displacement of the shaft 4 about the axis 17 with respect to that position of the shaft 4, the datum position, in which the longitudinal axis of the shaft 4 is coincident with a datum axis 20.

The signal supplied by the pick-off 18 is passed, after amplification by the amplifier 19, to energize an electric servo motor 21. The motor 21 is coupled through a clutch 22 to drive a pinion 23 that intermeshes with the sector gear 7. The drive exerted on the gear 7 by the motor 21 in this manner acts on the shaft 4, back via the link 6 and arm 5, to reduce the angular displacement about the axis 17 and restore the shaft 4 to its datum position with its longitudinal axis coincident with the axis 20.

The shaft 4 can be locked in its datum position by engagement of a lock 24, the lock 24 including a pin 25 which is slidably-mounted on the framework of the altimeter and which is adapted .to engage with a recess 26 provided on the longitudinal axis of the shaft 4 at the rear of the bearing 14. The pin 25 is urged by a spring 27 to slide along the axi into engagement with the recess 26 and lock the shaft 4 in its datum position, but is arranged to be held back from the recess 26, against the action of the spring 27, by an electrical solenoid 28. The solenoid 28 is connected in a supply lead 29 of the electrical power supply circuit of the altimeter so as to be energized only so long as electric power is being supplied .to each of the devices 18, 19, and 21. While it is energized, the solenoid 28 holds the pin back from the recess 26, thereby allowing the shaft 4 freedom for movement about the axis 17. As soon as there is any break in power supply to the devices 18, 19, and 21, and the solenoid 28 thereby becomes de-energized, the pin 25 is released to be urged rapidly into the recess 26 by the spring 27. The spring 27 is of a strength, and the recess 26 is o flared, as to ensure that whatever the extent, within the available limited range, of angular displacement of the shaft 4 about the axis 17, the shaft 4 is urged positively into its datum position and locked there by the pin 25.

In addition to controlling the lock 24, the solenoid 28 controls the clutch 22 such that the clutch 22 is engaged to transmit drive from the motor 21 to the pinion 23 only while the solenoid 28 is energized. Thus, as soon as there is any break in the electrical power supply to the devices 18, 19, and 21, and the solenoid 28 in consequence becomes de-energized, the clutch 22 is disengaged, thereby decoupling the pinion 23 from the motor 21.

The operation of the altimeter will now be described assuming firstly that electrical power is supplied to the devices 18, 19, and 21, and consequently that the solenoid 28 is energized maintaining the lock 24 disengaged and the clutch 22 engaged. In these circumstances deflection of the capsule 1, either expansion or contraction, produces via the link 2 and arm 3 angular displacement of the shaft 4 about both the longitudinal axis of the shaft 4 and the axis 17.

Angular displacement of the shaft 4 about its longi tudinal axis is opposed by the frictional resistance and other loading inherent in the coupling of the shaft 4 to the pointer 10 and counter 11 via the arm 5, the link 6, the gear 7, the pinion 8, and the differential gear 9. Additionally, such displacement is opposed by the added frictional resistance and loading imposed by the motor 21 coupled through the clutch 22 and pinion 23 to the sector gear 7. As a result therefore, the shaft 4 is angularly displaced about its longitudinal axis in accordance with only part of the thrust applied by the capsule 1 to the link 2. The remainder of this thrust, having moment about the axis 17, tends to produce angular displacement of the shaft 4 about the axis 17 against the restoring action of the leaf-springs 15. The result of any such angular displacement about the axis 17, as detected by the pick-off 18, is, however, to energize the motor 21 so that the sector gear 7 is driven via the pinion 23 to restore the shaft 4 to its datum position. Restoration of the shaft 4 to its datum position requires that the shaft 4 be driven to an angular displacement about its longitudinal axis that, with the shaft 4 in this position, is wholly consistent with the deflection of the capsule 1. The shaft 4 is accordingly driven by the motor 21 via the sector gear 7 to achieve an angular displacement about its longitudinal axis that is accurately representative of the deflection of the capsule 1. This in its turn ensures that the pointer 10 and counter 11 provide representations of altitude in true accord with the air-pressure sensed by the capsule 1, these representations being free from errors that would otherwise arise from frictional resistance and other loading.

In the event of failure of the power supply to any of the devices 18, 19, and 21, the solenoid 28 is de-energized. This releases the pin 25 to engage the recess 26 and thereby lock the shaft 4 in its datum position with its longitudinal axis coincident with the axis 20. In these circumstances, deflection of the capsule 1 can produce angular displacement of the shaft 4 about its longitudinal axis only. The de-energization of the solenoid 28 is also effective to disengage the clutch 22 so that the motor 21 is decoupled from the pinion 23 and the frictional resistance and other loading acting upon the capsule 1 via the shaft 4 is thereby reduced. The deflection of the capsule 1, acting against the remaining frictional resistance and other loading, produces angular displacement of the shaft 4 about its longitudinal axis, and this communicated via the sector gear 7, the pinion 8, and the differential gear 9, drives the pointer 10 and the counter 11 to provide representations of altitude. The representations of altitude provided in this case tend not to be so accurate as those provided when the shaft 4 is servo-driven,

since the frictional resistance and other loading imposed on the shaft 4 and acting thereby on the capsule 1, oppose deflection of the capsule 1 in true accord with the barometric air-pressure. Nonetheless, the altimeter continues in the event of electrical power failure to give reliable representations of altitude. These representations, although not of the high degree of accuracy obtained with servo operation, are of the same grade as obtained with a conventional, sensitive, mechanical altimeter and are therefore fully acceptable for use in safe flight of an aircraft.

It is desirable that the arm 5 be as close as possible to the intersection of the axis 17 with the longitudinal axis of the shaft 4. This reduces the restraint imposed upon the shaft 4 in opposition to the input thrust. Additionally there is also the fact that with the arm 5 spaced significantly from the intersection of the two axes 17 and 20, appropriate cancellation of angular displacement of the shaft 4 about the axis 17 may be prejudiced; in this respect it is undesirable that the pivot point of the .link 6 with the arm 5 shall at any time lie in line with the two other pivot points, namely the pivot point of the link 2 with the arm 3, and the pivot point of the shaft 4 at the fixed bearing 13.

It is normally necessary with the arrangement of FIG- URE 1, using one capsule, to provide some additional means for counteracting for effects of gravity and acceleration forces acting about the axis 17 and the longitudinal axis of the shaft 4.

In order to provide for self-balancing against the effects of gravity and acceleration forces, and also for greater accuracy in the representations of altitude provided by the altimeter while electrical power is not being supplied, two, or more, capsules may be used, these being coupled together to aid one another in driving the gearing to the pointer and counter 11. Each capsule in these circumstances may drive an individual rocker shaft, and a form of altimeter that includes such an arrangement will now be described briefly with reference to FIGURES 2 to 6.

Referring to FIGURES 2 to 6, two evacuated manometric capsules and 31 are mounted back-to-back on a central support 32 that is rigidly mounted between framework panels 33 and 34 within the external casing 35 of the altimeter. The capsules 30 and 31 are coupled, each via an individual link 36 and crank arm 37, to rocker shafts 38 and 39 respectively, so that each capsule 30 and 31 thereby tends to rotate its respective shaft 38 or 39 to an angular position dependent upon the sensed barometric pressure. The rocker shafts 38 and 39 extend lengthwise between the panels 33 and 34 and are each rotatably-mounted in jewel bearings 40 and 41 at either end. The bearing 40 at the foremost end of each shaft 38 and 39 is mounted in the panel 33, whereas the bearing 41 at the rearmost end is mounted in a respective V-shaped yoke-piece 42.

Each yoke-piece 42 is in itself rigid, but is resilientlyattached at its two ends to two pillars 43 respectively, that extend directly between the panels 33 and 34 to form part of the rigid framework of the altimeter. Each end of the yoke-piece 42 is hinged to the relevant pillar 43 by a short leaf-spring 44 so that the bearing 41 carried by the yoke-piece 42, together with the relevant shaft 38 or 39, is free for limited angular displacement in either direction normal to the longitudinal axis of the shaft 38 or 39 and the plane of the yoke-piece 42. (This form of mounting provided for each bearing 41 and using a V-shaped yoke-piece is preferred to the form using a pair of parallel leaf-springs and provided for the bearing 14 in the altimeter of FIGURE 1, since it enables a low spring-rate to be achieved in the directions of free movement with a high degree of rigidity normal thereto.)

The bearings 41 are each coupled by links 45 to opposite ends of a pivoted yoke-member 46 so that corresponding displacements of the bearings 41 (in opposite directions) aid one another in producing pivotal movement of the member 46. The member 46 is pivoted to the panel 34 by means of crossed-springs 47 which are carried by a block 48 that is rigidly mounted on the panel 34. The input shaft 49 of a synchro pick-off device 50 is coupled by a link 51 to the member 46 so that pivotal movement of the member 46 rotates the shaft 49 and (assuming electrical power is being supplied to the altimeter) causes an electric signal to be supplied by the device 50 to a servo amplifier 52 within the altimeter.

The member 46 carries a locking pin 53 that is adapted to be received by a flared recess 54 of a. pivoted arm 55. The arm 55 is urged by a spring 56 towards engagement with the pin 53, but is held out of engagement therewith against the action of the spring 56 by a solenoid 57 while electric power is being supplied to the altimeter via a power supply lead 58. The solenoid 57 is connected in the lead 58, and in the event of a break in supply of power, the solenoid 57 releases the arm 55 to engage the locking pin 53 in the recess 54. This restores to zero, and then inhibits, pivotal displacement of the member 46; the bearings 41 in thesecircumstances are held rigidly so that each shaft 38 and 39 is then capable of angular displacement about its longitudinal axis only.

The shafts 38 and 39 are coupled, via links 59 and 60 respectively, to an auxiliary shaft 61 so as to assist each other in rotating this shaft 61 to an angular position equivalent to the angular position of each shaft 38 and 39 about its longitudinal axis. The rotational position of the shaft 61 is conveyed via links 62 and 63 to rotate a common output shaft 64, the velocity ratio of the intercoupling provided in this manner between the shafts 61 and 64 being substantially constant at a value of approxirnately 1.85 throughout the working angle.

Referring now especially to FIGURE 6, the output shaft 64 drives a sector gear 65 that is coupled via differential gearing 66 to drive a pointer 67 and a digital drum counter 68. The pointer 67 is driven so that each complete revolution thereof indicates a change in altitude of one thousand feet, whereas the counter 68 is driven to provide a three-digit decimal indication of altitude in hundreds of feet. Correction for change in barometric pressure at ground-level is made by rotating a manuallyadjustable knob 69 that is coupled to the pointer 67 and counter 68 through the differential gearing 66, the setting of the knob 69 in millibars being indicated by a fourdigit decimal counter 70.

An electric servo motor 71 is coupled to the shaft 64 and sector gear 65 via a gear train 72 that includes a clutch 73. Engagement of the clutch 73 is controlled via a mechanical connection 74 from the solenoid 57, the clutch 73 being engaged only so long as electric power is supplied to the altimeter, and the solenoid 57 is energized. The motor 71 receives from the servo amplifier 52, in amplified form, the signal derived by the pick-off device 50 and provides drive to reduce this signal to zero. Accordingly, while electrical power is being supplied to the altimeter, the motor 71 acts to assist the capsules 30 and 31 to drive the pointer 67 and counter 68 to provide the appropriate, accurate indications of altitude. In the event of break in power supply, the capsules 3i) and 31 provide by themselves sufficient drive for maintaining reliable, albeit less-accurate, altitude indications.

It will be appreciated from the above description that the present invention, whilst being especially applicable to altimeters, is of wider application than this. The invention is of value generally where a small displacement produced by a sensor is required to be much expanded through gearing, and the power output: of the sensor is limited.

Taking the arrangement of 'FIGURE 1 is an example, it is not necessary for the thrust from the sensor or capsule 1 to be applied to the rotatably-mounted member or shaft 4 via a crank arm 3 as shown; it might alternatively be applied via a gear or pulley rigidly mounted on 7 the shaft4. The one essential is that the thrust be applied in such a way as to have moment about the second axis 17 as well as about the first, longitudinal axis of the shaft 4.

I claim:

1. An instrument for providing a representation dependent upon the value of a predetermined variable, comprising a member mounted for angular displacement about each of two mutually perpendicular axes, a sensor responsive to the value of said variable to apply to said member drive having moment about each of the two axes, means for deriving said representation from angular displacement of said member about a first of the two axes, and servo means for assisting the sensor to drive the said member to an angular displacement about said first axis in accord with the sensed value of said variable, said servo means being means for responding to angular displacement of said member about the second of the two axes to drive said member about said first axis to reduce the displacement about said second axis to zero.

2. An instrument according to claim 1 wherein said member is a shaft, said first axis being the longitudinal axis of the shaft.

3. An instrument according to claim 2 including gearing coupling the shaft to the means for deriving said representation, and wherein the servo means is coupled to apply drive to said shaft via part of the gearing.

4. An instrument according to claim 1 including locking means releasable to restrain said member from angular displacement about said second axis, and means for releasing the locking means in response to the condition in which the servo means is inoperative.

5. An instrument according to claim 4 wherein said locking means comprises two parts that are urged resiliently towards engagement one with the other, a first of said parts having a flared recess and the second of said parts includes a pin for engaging with the recess when the locking means is released.

6. An instrument according to claim 4 wherein the servo means is electrically-powered servo means, and a solenoid is responsive to break in supply of electrical power to the servo means to release said locking means.

7. An instrument according to claim 6 including a disengageable clutch for transmitting drive from the servo means to said member, and wherein said solenoid disengages the clutch in response to break in supply of electrical power to the servo means.

8. An instrument according to claim 1 wherein said sensor is a pressure-sensitive device, said pressure-sensitive device flexing in response to applied pressure for providing deflection dependent upon the value of the applied pressure, and wherein said instrument includes means coupling the pressure-sensitive device to said member to tend to displace said member angularly about both said first and second axes in accordance with the defiection.

9. An instrument according to claim 8 wherein said pressure-sensitive device is a manometric capsule.

10. An instrument according to claim 8 wherein said pressure-sensitive device is responsive to barometric pressure, and said representation is a representation of altitude.

11. An altimeter comprising a shaft, means mounting the shaft for angular displacement about two mutually perpendicular axes, a first of said axes being the longi- 8 tudinal axis of the shaft, a manometric capsule for providing deflection in accordance with barometric airpressure, means coupling the capsule to the shaft to displace the shaft angularly about the first and second axes in response to deflection of the capsule, indicator means coupled to the shaft to provide a representation of altitude in accordance with angular displacement of the shaft about said first axis, and an electrical servo arrangement for driving said shaft about said first axis to an angular displacement in accord with the deflection of the capsule, the servo arrangement including means for providing a signal dependent upon the angular deflection of the shaft about said second axis and a servo motor for driving the shaft about said first axis to reduce said signal to zero.

12. An altimeter according to claim 11 wherein said indicator means comprises a pointer and a digital counter.

13. An altimeter according to claim 11 including gearing coupling the indicator means to the shaft, and means coupling the servo motor to said shaft through part of the gearing.

14. An altimeter according to claim 13 wherein said means coupling the servo motor to the shaft includes a clutch, and wherein the altimeter includes means responsive to supply of electrical power to the servo arrangement to maintain said clutch in engagement only while power is being supplied.

15. An altimeter according to claim 11 including locking means that is releasable to urge the shaft to, and then maintain it in, a datum position of zero angular displacement about said second axis, and means for releasing the locking means in the event of break of supply of electrical power to the servo arrangement.

16. An altimeter comprising two elongated rocker shafts, means mounting each rocker shaft for angular displacement about its longitudinal axis and a transverse axis normal to the longitudinal axis, two manometric capsules that are responsive to barometric air-pressure, means mounting the two capsules back-to-back between the two rocker shafts, two mechanical linkages coupling the two capsules to the two shafts respectively for displacing each shaft angularly about its longitudinal and tfiansverse axes in accordance with the barometric air-pressure sensed by the respective capsule, gearing, indicator means coupled through said gearing to both rocker shafts to provide representation of altitude in accordance with the angular displacements of the shafts about their longitu dinal axes, and an electrical servo arrangement for driving the two rocker shafts about their longitudinal axes to angular displacements in accord with the barometric airpressure sensed by the capsules, said servo arrangement including means responsive to angular displacements of the two rocker shafts about their transverse axes and servo motor means for driving the two rocker shafts about their longitudinal axes to reduce to zero the displacements about the transverse axes.

References Cited UNITED STATES PATENTS 3,054,294 9/1962 Kishel 73386 3,283,583 11/1966 Angst 73-386 LOUIS R. PRINCE, Primary Examiner.

D. E. CORR, Assistant Examiner. 

1. AN INSTRUMENT FOR PROVIDING A REPRESENTATION DEPENDENT UPON THE VALUE OF A PREDETERMINED VARIABLE, COMPRISING A MEMBER MOUNTED FOR ANGULAR DISPLACEMENT ABOUT EACH OF TWO MUTUALLY PERPENDICULAR AXES, A SENSOR RESPONSIVE TO THE VALUE OF SAID VARIABLE TO APPLY TO SAID MEMBER DRIVE HAVING MOMENT ABOUT EACH OF THE TWO AXES, MEANS FOR DERIVING SAID REPRESENTATION FROM ANGULAR DISPLACEMENT OF SAID MEMBER ABOUT A FIRST OF THE TWO AXES, AND SERVO MEANS FOR ASSISTING THE SENSOR TO DRIVE THE SAID MEMBER TO AN ANGULAR DISPLACEMENT ABOUT SAID FIRST AXIS IN ACCORD WITH THE SENSED VALUE OF SAID VARIBLE, SAID SERVO MEANS BEING MEANS FOR RESPONDING TO ANGULAR DISPLACEMENT OF SAID MEMBER ABOUT THE SECOND OF THE TWO AXES TO DRIVE SAID MEMBER ABOUT SAID FIRST AXIS TO REDUCE THE DISPLACEMENT ABOUT SAID SECOND AXIS TO ZERO. 